Current sources and current source regulators are often used in mobile communication devices to provide a specific regulated current to several circuits and devices connected thereto. For example, regulators can be found in mobile communication devices, PDAs, several consumer devices and/or laptops and computers. In addition to a preferably stable and regulated output current, irrespectively of impedance changes of devices connected to an output of the regulated current source, the activation and deactivation of current source regulators become increasingly important.
FIG. 7 illustrates a known current source regulator having a main power transistor Mp whose output current is regulated using sensing transistor Ms1. A load LOAD connected to the output of power transistor Mp causes a voltage drop Vied. This voltage drop is detected and provided to a voltage equalizing network comprising transistors M2, M9, M6, M7 and M10.
A negative feedback loop regulates sensing current Is1 through sensing transistor Ms1 to a reference current given by Ib2−Ib1. An error is the difference between sensing current Is1 and the current Ib2. The error current is passed through a bias transistor M4 of the negative feedback loop. The error current is then compared with the current Ib1 to generate the control voltage signal Vg to drive the gate of sensing transistor Ms1 as well as of power transistor Mp. As a result, the negative feedback loop provides a stable output current Isrc for the load connected to the power transistor.
The maximum source gate voltage Vsg across power transistor Mp is given by the minimum control voltage Vg(min). This minimum control voltage Vg(min) is given by a sum of the saturation voltages of transistors M4 and M8. Vg(min) is given by the equation Vg(min)=Vsat(M4)+Vsat(M8). Consequently, the size of power transistor Mp depends on the minimum control voltage Vg(min).
If the saturation voltage Vsat of transistor M8 is reduced, the size of that transistor may increase correspondingly. Since transistor M8 supplies the reference current for the negative feedback loop, it should have a large length and at the same time large width to reduce the saturation voltage Vsat(M8). As an overall result, the area of the source regulator may increase as well.
In addition to these drawbacks a very high peak current may occur during activation of the regulator and sensing transistor Ms1 as well as of power transistor Mp.